- Inspecting the case and controls
- Voltage measurement
- How to use the built-in ohmmeter
- We measure the current in the circuit
- Continuity of the circuit and diodes
- Temperature measurement
- Checking field and bipolar transistors
- Special keys and functions
Universal digital meters, otherwise known as multimeters, have become indispensable helpers for many radio amateurs and electricians. Despite the abundance of modes, it is really easy to work with them, and today we offer the most complete instructions for using these devices..
Inspecting the case and controls
The vast majority of digital multimeters have a similar appearance and arrangement of control and display elements. It should be noted that the used ergonomics turned out to be very successful and convenient in work..
The main switch is located in the center – a disc with a longitudinal handle, which simultaneously serves as a position indicator with the desired mode. The modes and measurement ranges themselves are marked in the form of inscriptions in a circle from the switch. For convenience, adjacent modes are combined into groups (labels are surrounded by a frame), inside each you can switch between measurement limits.
Please note that the switch itself can be pass-through, that is, there are identical labels on both sides of the pointer. In other words, only half of the turnover is available for selection. Typically, such a circuit is used on a current clamp, while multimeters for the most part have a full 360? to select the desired mode.
In addition, the multimeter has an LCD display. Additional buttons can be located around it, including the display backlight and some additional functions. One or several additional buttons of the device can be located on the side edges of the device.
In the lower part of the body there are several holes with connectors for connecting probes. The connector labeled COM is a common negative contact for connecting a black probe. The rest of the connectors (usually two) are used to connect the red test lead: one for a wide range of measurements and one additional (signed A or ADC) for measuring high current values.
The easiest way is to measure voltage with a multimeter. There are two groups of measurements intended for this: DCV for DC and ripple current and ACV for AC. In the latter mode, the polarity of the probes can be disregarded, because the alternating current has no polarity as such.
The measurement limits for all multimeters are different, usually DC measure up to 1000 volts, and AC up to 700 or 750 volts. At the same time, there are several measurement ranges and, for example, when trying to measure a higher voltage in the limit up to 20 V, the device will simply give incorrect readings. But to measure the voltage obviously higher than the maximum limit is definitely not worth it, the device will simply fail. For some models, an excess of 100-200 V does not lead to death, but still it is not worth the risk.
When measuring DC and ripple currents, the polarity must be observed. This is a kind of opportunity to determine the polarity of an unknown source: if the probes are mixed up, a minus sign will appear in front of the voltage value. Just in case, we remind you that the voltage is measured with a parallel connection of the device.
How to use the built-in ohmmeter
In a multimeter, the resistance measurement function is considered the most popular. Usually the range group of the built-in ohmmeter is located at the bottom of the mode circle, indicated by the symbol? (Omega) and is divided into ranges from 100 or 200 Ohms to several hundred kOhms. Sometimes it is even possible to measure up to 10-20 MΩ through a separate connector for connecting a positive probe (External Unit) and connecting an external power supply.
When choosing different limits, the device continues to give correct readings, only the position of the separator point and, accordingly, the number of decimal places change. However, if the measurement limit is much less than the measured resistance, then the device will not give any readings at all..
If the resistance of the resistor being measured is unknown, it is best to move from the smallest limit to the highest. The accuracy of measuring resistances for most multimeters is low, about 1–2%. With a natural tolerance of resistors of 5-10%, the deviation from the declared value can be very significant. And the higher the range of measured values, the greater the error, this is especially true for the megohmmeter mode.
When measuring resistances, there are two more things to consider. First, with a discharged battery, the accuracy of readings can be extremely low. Secondly, if you are measuring very low resistances (units and tens of ohms), take into account the intrinsic resistance of the device and the probes, which is determined when the probes are short-circuited. Also, when measuring resistances, the most accurate value is indicated after 3-5 seconds, and not immediately.
We measure the current in the circuit
To measure the current strength, the device must be connected in series to the load circuit. The main connector for measurements is limited to rather small values - 0.2–0.5 A. It is possible to measure up to 10 A through the high-current connector, but the allowable voltage in the network decreases by 30–50% of the maximum measurement limit of the device. To measure current, the switch must be set to one of the positions of the DCA (constant) or ACA (variable) group. The latter type of measurement is found only in expensive instruments..
Note that there are different range groups for AC and DC current measurements. It is not scary to confuse them, the device will simply not show the correct values. Exceeding the maximum permissible current at a low-current connector leads to a blown fuse or failure of the device, at a high-current one – to a blown fuse.
Please note that in cheap Chinese multimeters, two positive connectors can be short-circuited and, of course, they will not be able to measure high currents. The rest is simple: choose the desired range, but at the same time it is better to move from the largest to the smallest. The device allows you to measure even microamperes, but the measurement accuracy of most digital devices is traditionally lame.
Continuity of the circuit and diodes
The diode symbol mode is designed to detect the voltage drop in a closed circuit. To check a diode, you need to touch its different leads, and then swap the probes. In one of the positions the display will show some readings, in the other the multimeter will not react in any way.
By the presence of readings, one can judge the polarity of the diode, in this position the black probe indicates the cathode. In fact, in this mode, the multimeter becomes a current source of 1 mA, and the reading on the display is nothing more than a voltage drop in mV. You can also ring the diodes in the ohmmeter mode: in one direction the current will flow, in the other not. However, it is the voltage drop that makes it possible to determine the characteristics of diodes without marking..
The audible continuity of the circuit in most models of multimeters is the smallest measurement range of the ohmmeter. If the resistance is below a certain threshold, which is usually 100 Ohm, the piezo emitter built into the device will turn on. Sometimes the sound appears with a noticeable delay.
Some multimeters are equipped with a thermocouple, thanks to which you can measure temperatures, including very high ones – up to 700-800? С. The thermocouple has a double plug and is installed in the COM connector and adjacent to it, or in a special pair of connectors marked with the letter “C”.
In the latter case, among the multimeter modes there is a similarly marked switch position. It will display the value in degrees Celsius on the display. If the multimeter does not have special connectors and mode, the temperature can be measured in DCV mode at the smallest limit. In this case, you need to use the table or graph of the dependence of thermo-EMF on temperature.
The measurement accuracy in the latter case will not be very high: the voltage recalculation will show not the actual temperature at the end of the thermocouple, but the difference between the measured object and the temperature of the multimeter itself. Compensation for this phenomenon is present in most devices with a special mode and connectors..
Checking field and bipolar transistors
Even the simplest multimeters can check transistors and determine their pinout. For bipolar transistors, the hFE mode and a special contact block are provided. The shoe is divided into two groups for P-N-P and N-P-N structure. Each contact is marked with the letters B (base), C (collector) and E (emitter).
The contacts are arranged in such a way that a three-terminal element with an unknown pinout could be quickly rearranged by turning it in different directions, and all combinations were tested. When the desired pinout is found, the device will display readings – the transfer coefficient of the transistor.
Please note that the pins of the pad are hidden deep enough that transistors with short legs, most likely, cannot be tested. Also, it will not be possible to check high-power transistors in this way: the current generated by the multimeter to open the junction is limited to a few microamperes.
Field-effect transistors are checked in diode continuity mode and the pinout must be reliably known. First, a negative probe is applied to the drain, and positive to the source. This checks the serviceability of the internal diode, with the reverse connection there will be no voltage drop.
If, without removing the negative probe from the drain, you touch the positive gate, the transistor will open, and the voltage drop between the drain and the source will become smaller and appear in both directions. You can close the transistor by touching the black shutter probe without removing the red one from the source. For P-channel transistors, the verification algorithm is similar, but at each stage the probes are swapped..
Special keys and functions
In conclusion, we will tell you about the special functions that are present in many multimeters, the cost of which exceeds 1300 rubles. The most important and frequently used is the HOLD key, which allows you to fix the current position on the display. One funny situation is connected with this: if the HOLD key is pressed, then when turned on, the multimeter will show anything on the display that can be regarded as a malfunction.
Also, in the display area, advanced devices have keys, by pressing which you can make the device display only the maximum, minimum or average readings instead of the actual ones. When various additional modes are activated, the corresponding mnemonic symbol is shown on the display.
The most advanced models also have functions for measuring the capacitance and frequency of the input signal, some multimeters even have a built-in oscilloscope and inductance measurement mode. Also, for expensive multimeters, there is no choice of the measurement limit on the rotary switch. Instead, the mode is selected, and the limit itself is toggled with the +/- buttons in the display area.